Method of determining transmission rate of control response frame for acknowledging data receipt in wireless lan

ABSTRACT

A method is provided for determining a transmission rate of a control response frame for acknowledging data receipt in a wireless local area network. The method includes obtaining transmission parameters of a transmitting station from a data transmission frame received from the transmitting station, searching a receiving station for transmission parameters which correspond to the transmission parameters of the transmitting station, determining the transmission rate of the control response frame according to transmission parameters of the receiving station if the transmission parameters corresponding to the transmission parameters of the transmitting station are found in the receiving station, and determining a maximum rate among a set of basic transmission rates to be the transmission rate of the control response frame if the transmission parameters corresponding to the transmission parameters of the transmitting station are not found in the receiving station.

BACKGROUND OF THE INVENTION

This application is a Continuation Application of U.S. patentapplication Ser. No. 12/486,185 filed Jun. 17, 2009, which is aContinuation Application of U.S. patent application Ser. No. 12/014,044filed Jan. 14, 2008, now U.S. Pat. No. 7,606,211, which is aContinuation Application of U.S. patent application Ser. No. 11/348,330filed on Feb. 7, 2006, now U.S. Pat. No. 7,339,916, which claimspriority from Korean Patent Application No. 10-2005-0016179, filed onFeb. 25, 2005 in the Korean Intellectual Property Office, and U.S.Provisional Patent Application No. 60/650,172, filed on Feb. 7, 2005 inthe U.S. Patent and Trademark Office, the disclosures of which areincorporated herein in their entirety by reference.

1. Field of the Invention

Methods consistent with the present invention relate to determining atransmission rate of a control response frame for acknowledging datareceipt in a wireless local area network (LAN).

2. Description of the Related Art

In wireless LAN environments, a medium access control (MAC) protocol ofa carrier sense multiple access/collision avoidance (CSMA/CA) mechanismis used. The CSMA/CA mechanism is designed to avoid collisions bytransmitting a signal when there is no data transmission through a cableof a network and sending data only after it is confirmed that the signalis transmitted without collisions.

The CSMA/CA mechanism works as follows: a terminal attempts to sense acarrier indicating that another terminal is already transmitting dataand, if the carrier is sensed, the terminal waits for a random period oftime. After the random period of time, the terminal attempts to sensethe carrier again. If no other carriers are sensed, the terminal startsto send data.

In the CSMA/CA mechanism, carrier sensing is performed using bothphysical carrier sensing and virtual carrier sensing. Physical carriersensing is performed at a physical layer (PHY), which senses whether areceived power exceeds a predetermined threshold and informs an MAClayer of whether a medium is “busy” or “idle” based on the sensingresult.

In virtual carrier sensing, if an MAC protocol data unit (MPDU) can beaccurately extracted from a received PHY packet data unit (PPDU), a“Duration/ID” field, which is one of a plurality of header fields of theMPDU, is interpreted. If the result of an interpretation indicates thata medium is “busy”, the medium is regarded as “busy” for a period oftime that the medium is expected to be used. As described above, whetheror not a medium is “busy” is determined using the two carrier sensingmethods, and the medium is not accessed if busy.

An MPDU/PHY service data unit (PSDU) received must be interpretednormally to effectively apply the virtual carrier sensing method to theCSMA/CA mechanism. In other words, for the virtual carrier sensingmethod, a value of an MAC header must be read normally. If errors occurdue to an unstable channel when data is transmitted at a hightransmission data rate, or if a receiving station cannot handle the hightransmission data rate, the MPDU/PSDU cannot be interpreted. In thiscase, virtual carrier sensing is not possible and thus, the CSMA/CAmechanism is ineffective.

FIG. 1 illustrates an IEEE 802.11a-based legacy PPDU frame format.Referring to FIG. 1, if preamble and signal fields in the PPDU frameformat are received normally, duration information of a data field canbe estimated using rate and length information included in the signalfield. Hence, information contained in the preamble and signal fields isuseful for a clear channel assessment (CCA) mechanism.

If the preamble and signal fields in the PPDU frame being received areinterpreted but a frame check sequence (FCS) error occurs at a receivingstation, an MAC layer controls the receiving station to wait for anextended interframe space (EIFS), which is 94 μs in the case of IEEE802.11a, not a DCF interframe space (DIFS), which is 34 μs in the caseof IEEE 802.11a, and backs off.

In other words, if high throughput (HT) stations and legacy stations(802.11a/b/g) with different transmission capabilities coexist on awireless LAN, the legacy stations cannot interpret an HT frame. Thus,the MAC layers respectively included in the legacy stations fail toaccurately carry out virtual carrier sensing and rely only on physicalcarrier sensing.

Even if the preamble and signal fields of an HT PPDU frame are formattedsuch that the legacy stations can interpret them, the legacy stationscannot accurately interpret the data field. Thus, the legacy stationshave the FCS error and regard the HT PPDU frame as a flawed frame. Then,the MAC layers control the respective legacy stations to wait for theDIFS. On the other hand, stations that can handle a high transmissionrate, i.e., the HT stations, can carry out accurate virtual carriersensing. Thus, the HT stations wait for the DIFS as usual.

Since EIFS=short interframe space (SIFS)+TACK (at the lowest datarate)+DIFS, stations that cannot handle the data rate, i.e., the legacystations with lower transmission capabilities than the HT stations, aregiven lower medium access priorities than the HT stations. As a result,medium access fairness for all stations, which is maintained by adistributed coordination function (DCF), cannot be secured.

However, the medium access fairness can be secured when a legacyacknowledgement (ACK) frame is used for acknowledging data transmissionon the wireless LAN as illustrated in FIG. 2.

An HT transmitting station HT SRC transmits data to an HT receivingstation HT DEST using the HT PPDU frame format. Then, the HTtransmitting station HT SRC and the HT receiving station HT DEST waitfor a SIFS. After the SIFS, the HT receiving station HT DEST transmitsan ACK frame in a legacy format to the HT transmitting station HT SRC toconfirm the receipt of data.

When the HT transmitting station transmits the ACK frame in the legacyformat, other legacy stations (802.11a-based stations in FIG. 2) as wellas the HT receiving station interpret the data field normally. Thus, allstations wait for a DIFS. Consequently, all stations can compete formedium access on an equal footing.

In an IEEE 802.11 standard, if a receiving station supports a frametransmission rate used by a transmitting station, the frame transmissionrate is determined to be a transmission rate of a control responseframe. If the receiving station does not support the frame transmissionrate, the maximum transmission rate among a set of basic transmissionrates supported by the wireless LAN is determined to be the transmissionrate of the control response frame.

As described above, on the wireless LAN where HT stations and legacystations with different transmission capabilities coexist, a legacy ACKframe is used for acknowledging data receipt. Accordingly, atransmission rate must be determined using a method different from aconventional method.

SUMMARY OF THE INVENTION

The present invention provides a method of determining a transmissionrate of a control response frame for acknowledging data receipt, whichenables HT stations and legacy stations with different transmissioncapabilities to access a medium on an equal footing in a wireless LANenvironment where the stations coexist, thereby complementing a carriersensing method.

According to an aspect of the present invention, there is provided amethod of determining a transmission rate of a control response framefor acknowledging data receipt in a wireless local area network wherehigh throughput stations and legacy stations with different datatransmission capabilities coexist. The method includes: (a) obtainingtransmission parameters of a transmitting station from a datatransmission frame received from the transmitting station; (b) searchinga receiving station for transmission parameters which correspond to theobtained transmission parameters of the transmitting station; and (c)determining the transmission rate of the control response frame based onthe search result.

If the transmission parameters corresponding to the obtainedtransmission parameters of the transmitting station are found in thereceiving station, the transmission rate of the control response frameis determined according to the obtained transmission parameters of thereceiving station, and if the transmission parameters corresponding tothe obtained transmission parameters of the transmitting station are notfound in the receiving station, a maximum rate among a set of basictransmission rates included in the receiving station is determined to bethe transmission rate of the control response frame.

In operation (a), if the data transmission frame received from thetransmitting station is a high throughput PHY packet data unit frame,the transmission parameters of the transmitting station are obtainedfrom a signal field of the high throughput PHY packet data unit framewith reference to a modulation coding scheme index.

In operation (a), if the data transmission frame received from thetransmitting station is a legacy PHY packet data unit frame, thetransmission parameters of the transmitting station are obtained from asignal field of the legacy PHY packet data unit frame with reference toa rate field.

In operation (b), the receiving station is searched for transmissionparameters corresponding to a legacy format among the transmissionparameters of the receiving station, which correspond to the obtainedtransmission parameters of the transmitting station.

Operation (c) includes: (c1) determining whether the transmittingstation supports the transmission rate of the control response framedetermined according to the obtained transmission parameters of thereceiving station if the transmission parameters corresponding to theobtained transmission parameters of the transmitting station are foundin the receiving station; and (c2) determining the transmission rate ofthe control response frame based on the result of determination.

In operation (c2), if the receiving station supports the determinedtransmission rate, the determined transmission rate of the controlresponse frame is used, and if the receiving station does not supportthe determined transmission rate, the maximum rate among the set ofbasic transmission rates is determined to be the transmission rate ofthe control response frame.

The transmission parameters include a number of spatial streams, amodulation scheme, and a coding rate. The control response frame has alegacy format. The control response frame is a clear-to-send frame or anACK frame. The high throughput stations include systems that usemultiple-input-multiple-output technology. The high throughput stationsinclude systems that use channel bonding.

The legacy stations include systems that meet an IEEE 802.11 a/b/gstandard.

According to another aspect of the present invention, there is provideda computer-readable recording medium on which a program is recorded forexecuting a method of determining a transmission rate of a controlresponse frame for acknowledging data receipt in a wireless local areanetwork where high throughput stations and legacy stations withdifferent data transmission capabilities coexist.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and/or other aspects of the present invention will become moreapparent by describing in detail exemplary embodiments thereof withreference to the attached drawings in which:

FIG. 1 illustrates an IEEE 802.11a-based legacy PPDU frame format;

FIG. 2 illustrates data and legacy ACK frame transmissions on a wirelessLAN where HT stations and legacy stations with different transmissioncapabilities coexist;

FIG. 3 illustrates an HT PPDU frame format;

FIG. 4 is a flowchart illustrating a method of determining atransmission rate of a control response frame for acknowledging datareceipt in the wireless LAN where HT stations and legacy stations withdifferent transmission capabilities can coexist according to anexemplary embodiment of the present invention; and

FIG. 5A and FIG. 5B illustrate a table of a modulation coding scheme(MCS) field that defines modulation and coding schemes in the HT PPDUframe format.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

The present invention will now be described more fully with reference tothe accompanying drawings, in which exemplary embodiments of theinvention are shown. The invention may, however, be embodied in manydifferent forms and should not be construed as being limited to theexemplary embodiments set forth therein; rather, these exemplaryembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the concept of the invention to thoseskilled in the art.

FIG. 4 is a flowchart illustrating a method of determining atransmission rate of a control response frame for acknowledging datareceipt in a wireless LAN where HT stations and legacy stations withdifferent transmission capabilities can coexist according to the presentinvention. Referring to FIG. 4, to determine the transmission rate ofthe control response frame for acknowledging data receipt, a receivingstation obtains transmission parameters from a data transmission framereceived from a transmitting station (S100).

If the data transmission frame received from the transmitting station isan HT PPDU frame, the transmission parameters are obtained from a signalfield of the HP PPDU frame with reference to a modulation coding scheme(MCS) index. If the data transmission frame received from thetransmitting station is a legacy PPDU frame, the transmission parametersare obtained from a signal field of the legacy PPDU frame with referenceto a rate field.

The transmission parameters used in the present exemplary embodimentinclude a number of spatial streams, a modulation scheme and a codingrate. Different transmission parameters may be used in otherembodiments.

Next, the receiving station searches for transmission parameterscorresponding to the obtained transmission parameters of thetransmitting station (S200). In operation S200, the receiving stationsearches for transmission parameters corresponding to a legacy formatamong its transmission parameters corresponding to the obtainedtransmission parameters of the transmitting station. In other words,referring to FIG. 5A and FIG. 5B, the receiving station searches fortransmission parameters satisfying the condition that the number ofspatial streams is one (i.e., MCS indices of 0 through 7). Then, amongits transmission parameters satisfying the condition, the receivingstation searches for transmission parameters that also satisfy othertransmission parameters of the transmitting station.

It is determined whether the receiving station has transmissionparameters corresponding to the obtained transmission parameters of thetransmitting station (S300). If it is determined that the receivingstation has the transmission parameters corresponding to the obtainedtransmission parameters of the transmitting station, a transmission rateof a control response frame is determined according to the transmissionparameters of the receiving station, which correspond to the obtainedtransmission parameters of the transmitting station (S400).

If it is determined that the receiving station does not have thetransmission parameters corresponding to the obtained transmissionparameters of the transmitting station, operation S700 is performed.That is, the maximum transmission rate among a set of basic transmissionrates supported in a wireless LAN environment is determined to be thetransmission rate of the control response frame.

If it is determined that the receiving station supports the determinedtransmission rate of the control response frame determined in operationS400 (S500), the receiving station transmits the control response frameat the determined transmission rate (S600).

If it is determined that the receiving station does not support thedetermined transmission rate, the maximum transmission rate among a setof basic transmission rates supported in the wireless LAN environment isdetermined to be the transmission rate of the control response frame(S700).

In the present exemplary embodiment, the control response frame may be aclear to send (CTS) frame or an ACK frame.

FIG. 5A and FIG. 5B illustrate a table of an MCS field that definesmodulation and coding schemes in the HT PPDU frame format. Referring toFIG. 5A and FIG. 5B, 16 bits are used for the MCS field, which includesfields indicating an MCS index, a number of spatial streams, amodulation scheme, a coding rate, and a transmission rate. In thepresent exemplary embodiment, the MCS table of FIG. 5A and FIG. 5B isused. However, the MCS table may be user defined.

The method of determining the transmission rate of the control responseframe to acknowledge data receipt illustrated in FIG. 4 will now bedescribed with reference to the MCS table of FIG. 5A and FIG. 5B byusing an example.

It is assumed that a receiving station has received data from atransmitting station at a transmission rate corresponding to MCS index14. With reference to the MSC index 14, the receiving station determinesthat the number of spatial streams is two, the modulation scheme is64-QAM, the coding rate is ¾, and the transmission rate is 108 from thesignal field of the HT PPDU frame indicated in the received data.

The receiving station searches for a transmission rate that satisfiesthe conditions that the number of spatial streams is one, the modulationscheme is 64-QAM, and the coding rate is ¾, and determines that thetransmission rate 54 satisfies the conditions. If no transmission ratesatisfying these conditions is found, the maximum transmission rateamong a set of basic transmission rates supported in the wireless LANenvironment is determined to be the transmission rate of the controlresponse frame.

It is determined whether the receiving station supports the transmissionrate 54. If it is determined that the receiving station supports thetransmission rate 54, the receiving station maintains the transmissionrate 54. If it is determined that the receiving station does not supportthe transmission rate 54, the maximum transmission rate among the set ofbasic transmission rates supported in the wireless LAN environment isdetermined to be the transmission rate of the control response frame.

As described above, according to a method of determining a transmissionrate of a control response frame for acknowledging data receipt in awireless LAN, medium access fairness can be secured in a wireless LANenvironment where HT stations and legacy stations with differenttransmission capabilities coexist. In addition, the transmission rate ofthe control response frame can be determined appropriately for thewireless LAN environment.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1. A high throughput station apparatus in a wireless local area networkwherein at least one high throughput station and at least one legacystation with different data transmission capabilities respectivelycoexist, the station comprising a processor being configured to obtain amodulation scheme and a coding rate of a data transmission frame of ahigh throughput format received from a transmitting high throughputstation, and determine a transmission rate of a control response frameto be a highest rate in a set of basic transmission rates of a receivingstation when a transmission rate corresponding to the obtainedmodulation scheme and the obtained coding rate of a legacy format is notsupported by the receiving station.
 2. The high throughput stationapparatus of claim 1, wherein the modulation scheme and the coding rateof the data transmission frame are obtained from a signal field of thedata transmission frame with reference to a modulation coding schemeindex.
 3. The high throughput station apparatus of claim 1, the controlresponse frame is a clear-to-send frame or an acknowledgement frame. 4.The high throughput station apparatus of claim 1, wherein the highthroughput station transmits and receives data usingmultiple-input-multiple-output technology.
 5. The high throughputstation apparatus of claim 1, wherein the high throughput stationtransmits and receives data using use channel bonding.
 6. The highthroughput station apparatus of claim 1, wherein the legacy stationtransmits and receives data in compliance with an IEEE 802.11 a/b/gstandard.